The present invention relates to an optical module used in the optoelectronic field and optical communication field, and a manufacturing method for the optical module.
Recently, for miniaturizing optical circuits, various researches and developments have been conducted on silicon wire optical waveguides using SOI (Silicon On Insulator) substrates and photonic crystal waveguides. Problems are posed in connection between these optical waveguides and optical fibers, light-emitting devices, light-receiving devices, and the like in terms of the respective mode field sizes (diameters). These optical waveguides have mode field sizes on the submicron order, whereas optical fibers and the like have mode field sizes on the order of several microns. It is therefore difficult to efficiently make direct connection between an optical waveguide and a general optical fiber or the like having a large mode field size. In order to connect them with a low loss, a mode field size must be converted.
For this purpose, various kinds of mode field size conversion structures have been proposed. For example, on an SOI substrate on which the first optical waveguide formed from a silicon wire is formed, the second optical waveguide made of a quartz-based material or polymer which is to be connected to the first optical waveguide is formed, and the second optical waveguide and the first optical waveguide having a tapered distal end are made to overlap each other, thereby realizing high-efficiency mode field size conversion (for example, T. Shoji et al, “Optical Interconnecting Structure of Si Waveguide on SOI Substrate”, 30a-YK-11 Extended No. 3 Abstracts (The 48th Spring Meeting, 2001), The Japan Society of Applied Physics and Related Societies).
FIGS. 25A and 25B show a conventional optical waveguide having a mode field size (spot size) conversion structure. Referring to FIGS. 25A and 25B, reference numeral 10 denotes a first optical waveguide formed from a silicon wire; 11, a mode field size conversion structure; 12, a second optical waveguide connected to the first optical waveguide; 13, a silicon substrate; 14, an under cladding made of silicon oxide and formed on the silicon substrate 13; 16, a wire-like core made of silicon and formed on the under cladding 14; 17, a tapered portion which is made of silicon and extends from the core 16; and 18, a core made of a polymer and placed on the tapered portion 17. The core 16, tapered portion 17, and core 18 are arranged on the silicon substrate 13 and under cladding 14 as a common substrate, thereby connecting the first optical waveguide 10 to the second optical waveguide 12 through the mode field size conversion structure 11.
When light in the 1.55-μm band, which is used most for optical communication, is to be passed, the height and width of a cross section of the core 17 constituting the first optical waveguide 10 are about 0.3 μm each. The core 18 of the second optical waveguide 12 which is connected to the first optical waveguide 10 has a refractive index higher than the under cladding 14 by few %. Both the height and width of a cross section of the core 18 are about several μm. Reference numeral 16 denotes the core made of silicon and having the tapered portion 17. This core has a length of 200 μm, and the width of the tapered distal end portion is 0.06 μm. The core 16 and tapered portion 17 are formed by electron beam lithography and etching. The core 18 made of a polymer is formed by photolithography.
In order to connect an optical fiber to the conventional optical module shown in FIGS. 25A and 25B with a low loss, a mode field diameter F of the second optical waveguide to be connected to the first optical waveguide in the form of a wire is required to be near the mode field diameter (9 μm) of the optical fiber.
In the conventional optical module shown in FIGS. 25A and 25B, however, since air having a refractive index of 1 serves as an over cladding, the refractive index difference between the air and the core 18 of the second optical waveguide 12 is large. For this reason, the core size of the second optical waveguide 12 which satisfies the single mode condition cannot be larger than 3 μm square.
In addition, since there is no over cladding layer around the core 16 of the first optical waveguide 10 made of a silicon wire, the core 16 of the optical waveguide 10 tends to be damaged, resulting in an increase in propagation loss.
In the optical module shown in FIGS. 25A and 25B, in order to improve the mode field size conversion efficiency, the width of the tapered distal end is required to be 0.1 μm or less, ideally about 0.06 μm. Such micro fabrication demands highly sophisticated lithographic techniques such as electron beam drawing and etching techniques. It is therefore difficult to economically process tapered portions.